title: On two different objectives of the concepts of ionic radii.
abstract: Experimentally and theoretically derived interatomic distances (D) and ionic radii (R) of more than a hundred monomeric (AX), dimeric (A2X2, ABXY), and crystalline ([AX]) alkali halide species (A=Li, Na, K, Rb, Cs, Fr; X=H, F, Cl, Br, I, At) have been analyzed. Chemists use the word "atomic radius" for two antithetic concepts. Let D(CiEE'jj') be the "billion" of distances i between two adjacent atoms in the millions of known compounds C from a hundred different elements E in bonding states j. The common chemical aim is partitioning D approximately into increments R(Ej) +R(E'j'). This can be achieved with a few (say <thousand) predictive constants R(Ej). An antipodal aim is specifying in hindsight an electron density feature in the "billion" of different bonds i, by partitioning them into "two billions" of exact bonded radii R(E)(Ci) + R(E')(Ci). The constant incremental and the variable bonded radii concepts with the same generic name are useful in different fields of research. Different concepts should be well distinguished, since they have different meaning, different numerical values, and different purposes.
authors.affiliation: Theoretical and Computational Chemistry Laboratory, Department of Chemistry, Tsinghua University, Beijing, 100084 (P. R. China), Fax: (+86) 10-62797472.
authors.forename: Jian-Biao
authors.initials: JB
authors.lastname: Liu
authors.forename: W H Eugen
authors.initials: WH
authors.lastname: Schwarz
authors.forename: Jun
authors.initials: J
authors.lastname: Li
keywords: alkali halides; atomic radii; atoms in molecules; chemical concepts; electron density